(1) Field of the Invention
The present invention relates to a thermal head and a thermal printer using the thermal head, in particular, it relates to a thermal head which can be improved to a real edge thermal head and which can improve the printing quality when the thermal head is used for a printer of a thermal transcription system, the manufacturing method of the thermal head, and a thermal printer using the thermal head.
(2) Description of the Prior Art
In a thermal head to be mounted on a thermal printer, a plurality of heating elements are disposed in a straight line on a substrate and one of the heating elements is selectively energized to be heated one after another based on a desired printing information; thereby, in a thermosensible printer, a thermosensible recording paper is colored and in a thermal transcription printer, ink on an ink ribbon is partially melted and it is transcribed on a plain paper for printing.
FIG. 7 shows a general thermal head of a conventional type. A heat insulating layer 2 composed of glass, etc. is formed on a substrate 1 which is composed of an insulating material such as alumina and the upper surface of the heat insulating layer 2 is formed to be a part of a circular arc, and a plurality of heating resistors 3 are formed in a straight line on the top 2a of the insulating layer 2, and these heating resistors are formed by photolithography or etching after a material for heating resistors composed of Ta.sub.2 N or the like is attached on the surface of the heat insulating layer 2 by vapor deposition or sputtering. A common electrode 4a to be connected to heating resistors 3 is formed by lamination on a side of the upper surface of the heating resistors 3 and individual electrodes 4b for energizing each of the heating resistors independently are formed by lamination on the other side of the heating resistors 3. The above-mentioned common electrode 4a and individual electrodes 4b are composed of Au or Cu, for example, and after they are attached by vapor deposition, sputtering, etc., they are formed into desired patterns by etching, etc.
A protective layer 5 which is 7 to 10 .mu.m thick for protecting heating resistors 3 and electrodes, 4a and 4b, is formed over the heating resistors 3, the common electrode 4a, individual electrodes 4b, an exposed part on the surface of the substrate 1, and the surface of the heat insulating layer 2. The protective layer 5 is formed to cover all the surfaces except the terminal portions of the electrodes, 4a and 4b.
In the case of such a conventional thermal head, it has been needed to make the resistance of the common electrode 4a low by forming the width of the electrode wide, so that, as shown in FIG. 7, the heating portions 3a of the heating resistors 3 are disposed in the central part or in the vicinity of the edge portion of the thermal head substrate 1, and the distance from the heating portions 3a of the heating resistors 3 to the edge portion of the substrate 1 has been more than several mm.
In recent years, the demand for the real edge constitution of a thermal head, that is, the demand for disposing the heating resistors 3 of a thermal head in a closer position to the edge of a substrate 1 is increasing, and it is urgently needed to sharply curtail the space on the side of the common electrode 4a on the substrate 1. The real edge constitution of a thermal head offers merits as shown below: it decreases the loss in the abutting pressure of a head against a platen and improves printing energy efficiency, and in the case of a thermal transcription printer using an ink ribbon, inks of a variety of kinds, from the ink of a wax group to the ink of a resin group, can be used, and the printing quality can be remarkably improved when a rough paper (a paper having a low degree of smoothness on the printing surface) is used.
When the real edge constitution of a thermal head having an edge distance of less than 0.2 mm is contrived, the space for the disposition of a common electrode 4a becomes very small, so that the width dimension of the common electrode 4a has to be extremely thin; in the result, the resistance of the common electrode 4a is so increased as to be regarded as a resistor, and the difference in voltage drops between both end parts and the central part becomes large. In some case, the current capacity of the common electrode 4a becomes insufficient and when the heating resistors are energized, a trouble such as a breakdown of the common electrode 4a can occur, and it has been very difficult to manufacture a real edge head having high utility.
As other types of electrodes of a thermal head (not shown in a drawing) in which real edge constitution is contrived, there are, for example, turnup electrodes or comb type electrodes in which leads of a common electrode and individual electrodes are led to the same side; in such a case, however, when the resolution is assumed to be 300 dpi (dots per inch), the working precision equivalent to that in a case of 600 dpi is required, and in the similar way, when the resolution is assumed to be 400 dpi, the working precision equivalent to that in a case of 800 dpi is required. In the above case, there are demerits as shown below: manufacturing mandays are increased, manufacturing yield is lowered, reliability of products is lowered, and the manufacturing cost is increased.
Further, there is a thermal head in which a common electrode is formed onto the back surface from the edge portion of a thermal head substrate; however, there are demerits in such a thermal head as shown below: since electrodes are formed after division and polishing of a substrate, a lot of manufacturing mandays are required, manufacturing efficiency is lowered, and the reliability in a real edge constitution within 0.2 mm is extremely low.
In the case of an edge surface head, the edge surface of a substrate is polished to form a heat insulating layer on it and then heating elements are formed on the layer. Therefore, similar to the above case, a lot of mandays are needed, and mass productivity is lowered when a real edge constitution is contrived, which raises the manufacturing cost.
In consideration of these points, there has been a thermal head in which real edge constitution is contrived in adopting a multilayer wiring constitution for a common electrode in the heating portion. In this case, a conductive layer composed of a metallic material is formed on a heat insulating layer, and a layer insulating layer composed of SiO.sub.2, etc. is laminated on the conductive layer with a technique such as etching, and then the layer insulating layer is partially removed with a technique of photolithography, and heating resistors are laminated over it for the purpose of connecting the conductive layer and the heating resistors electrically. The layer insulating layer and the conductive layer are formed into a lamination structure and disposed right under the heating resistors which are heated to a high temperature.
In the case of a thermal head having a multilayer wiring constitution as described in the above, when a desired heating resistor is energized through an individual electrode based on a desired printing signal, electricity can be supplied through the conductive layer beside the common electrode which is formed to have an extremely narrow width for the purpose of obtaining a real edge constitution, so that the effective resistance of the common electrode is kept low and the occurrence of partial voltage difference in the heating resistors or the occurrence of insufficiency in the current capacity of the common electrode can be prevented, which makes it possible to perform printing of high quality.
In the case of a conventional thermal head, however, there are problems as described below: a layer insulating layer is formed on a conductive layer and besides the layers are formed right under the heating resistors which are heated to a high temperature, so that the stress between layers becomes large and the reliability in the adhesive force between layers against a heat shock is lowered much; the layer insulating layer is formed with etching, so that there can be produced a difference in level between the surface of the layer insulating layer and the surface of the conductive layer, which may cause bad connection between the heating resistors and the conductive layer; and moreover when the layer insulating layer is formed with a vapor deposition system such as a sputtering method, a pin hole can occur in the layer insulating layer caused by a foreign substance, etc., which may cause an insulation trouble.
When the thickness of the layer insulating layer is increased, a problem can occur in that the stress imbalance between layers grows larger and also the reliability can be lowered with bad connection, etc. caused by the increase in the level difference produced in the etching process.